There are various processes known within the printing industry for transferring designs by means of print originals to paper or films, for example. One possibility is that known as flexographic printing.
In the flexographic printing process, flexible printing plates are bonded to printing cylinders or printing sleeves. Such plates consist, for example, of a polyethylene terephthalate film (PET film) on which there is applied a photopolymer layer into which the appropriate print relief can be introduced by exposure of the print elements and subsequent washing-out of the non-print elements. The plate is then bonded to the printing cylinder or printing sleeve by way of the PET film. For the bonding, generally speaking, double-sided pressure-sensitive adhesive tapes are used, on which very stringent requirements are imposed. For the printing operation, the pressure-sensitive adhesive tape is required to have a certain hardness, but also a certain elasticity. These properties must be set very precisely in order that the printed image produced yields the desired outcome in accordance with the requirements. Stringent requirements are likewise imposed on the pressure-sensitive adhesive (PSA), since the peel adhesion ought likewise to be sufficient so that the printing plate does not detach from the double-sided pressure-sensitive adhesive tape, or the pressure-sensitive adhesive tape from the cylinder or sleeve. This must be so even at elevated temperatures of 40 to 60° C. and at relatively high printing speeds. In addition to this property, however, the PSA must also possess reversible adhesion properties, to allow the printing plates to be detached again after the printing operations (in that situation, the adhesive bond of the pressure-sensitive adhesive tape to the printing cylinder or printing sleeve, and also the adhesive bond to the plate, must be able to be parted without residue, in order to ensure that both components can be used again). This detachability ought also to exist even after bonding over a relatively long period (up to 6 months). It is desirable, moreover, for it to be possible to remove the pressure-sensitive adhesive tape and especially the printing plate without destruction thereof, and also without great application of force, since in general the printing plates are used a number of times. Furthermore, there should be no residues on the printing plate and on the cylinder or sleeve. In summary, therefore, very exacting requirements are imposed on the double-sided pressure-sensitive adhesive tapes suitable for this use.
Residue-free redetachability is a problem especially in the case of polar substrates such as steel, for example, since here it has been found that the peel adhesion increases considerably over the course of time. For the purposes of the present specification, in relation to surfaces, the terms “polar” and “high-energy”, i.e., having a high surface energy (SE), are equated, as are the terms “nonpolar” and “low-energy”, since this simplifying model has become established in the art. The finding that lies behind this is that polar dipole forces are comparatively strong relative to what are called “disperse” or nonpolar interactions, which are built up without participation of permanent molecular dipoles. The basis for this model of interfacial energy and interfacial interactions is the idea that polar components interact only with polar components, and nonpolar components only with nonpolar components.
This energy and its components are often measured by measurement of the static contact angles of different test liquids. The surface tensions of these liquids are assigned polar and nonpolar components. From the contact angles observed between the droplets and the test surface, the polar and nonpolar components of the surface energy for the test surface are ascertained. This can be done, for example, according to the OWKR model. One alternative method customary industrially is the determination using test inks according to DIN ISO 8296.
Examples of possible pressure-sensitive adhesives include those based on natural rubber, as documented by EP 760 389 A, for example. Also employed for the stated purpose, however, are pressure-sensitive adhesive tapes having polyacrylate-based PSAs. Accordingly, for example, WO 03/057497 A describes an acrylate PSA based on block copolymer for the stated application. WO 2004/067661 A discloses a pressure-sensitive adhesive tape with a PSA based on at least 49.5 wt % of a soft acrylic monomer (Tg<−20° C.); of a hard, cyclic or linear (meth)acrylic ester monomer (Tg>30° C.); at least 10 wt % of hard (meth)acrylic acid/ester monomers (Tg>30° C.) and at least 0.5 wt % of functionalized hard (meth)acrylic acid/ester monomers (Tg>30° C.), the PSA being produced in a two-stage method.
A further disadvantage of many PSAs known from the prior art for the adhesive bonding of printing plates is manifested especially when the bonded printing plates are to be cleaned to remove the printing ink. This is normally brought about by using the solvents that also serve as solvents for the inks themselves, in large amounts, for washing and removing the inks from the plates. Inevitably in this procedure, there is creepage of solvent below the edges of the bond of the plate on the pressure-sensitive adhesive tape, and of the edges of the adhesive tape on the printing cylinder or printing sleeve. This results in detachment of the bond (of the plate to the adhesive tape and of the adhesive tape to the cylinder or sleeve), since the adhesives of the pressure-sensitive adhesive tape lose the necessary adhesion. The lifted edges (“flags”) produced as a result of this lack of solvent resistance are simultaneously printed in the process, as a result of which a flawed printed image (generally known as a misprint) is produced, if there are not, indeed, mechanical problems with the flags in the printing apparatus and hence system outages. In practice, therefore, the bonds on printing plates mounted with prior-art adhesives have to be protected from the solvent by sealing of the respective edges with single-sided pressure-sensitive adhesive tapes or with liquid adhesives or hotmelt adhesives.
This additional sealing operation implies a significant extra expense, and the risk exists of damaging the expensive printing plates on demounting, particularly where liquid adhesives or hotmelt adhesives are used.
EP 2 226 372 A1 discloses an acrylate-based PSA for the bonding of printing plates to cylinders or sleeves that has a high acrylic acid fraction of between 8 and 15 wt %. Further monomers are linear and branched acrylic esters, and are present in a defined ratio to one another. Using such an adhesive, the requirements in terms of edge lifting behaviour and solvent resistance are met very well. PSAs with a high acrylic acid fraction, however, are prone to strong peel increase on polar substrates, such as steel, which is commonly the material for printing cylinders. Also being used increasingly are plastic sleeves, very often based on polyurethane. The adhesives on the printing sleeve side must adhere both to steel and to low-energy sleeve surfaces, and this poses an additional challenge in the development process. This problem also arises with the adhesive of EP 2 226 372 A1, particularly if it is used on the side of the adhesive tape facing the printing cylinder or printing sleeve. Demounting such adhesives from such substrates, therefore, entails problems; very high demounting forces arise, and the adhesive tape used may fracture, or residues remain on the substrate.
In order to provide a PSA which, even under the influence of solvents, ensures effective and reliable bonding to material common in flexographic printing, such as to PET (polyethylene terephthalate) in particular, but which nevertheless is still redetachable even after a prolonged time and even from highly polar substrates, such as the surfaces of steel printing cylinders or the surfaces of certain printing sleeves comprising polar plastic surfaces, for example, where the PSA ought preferably to be suitable in particular for the reliable bonding of printing plates, and where, for an adhesive tape with the PSA, the stability of the adhesive tape assembly, particularly the reliable anchoring of the PSA on foam carriers—such as polyolefinic foams—is to be ensured, WO2014/001096 A1 discloses an acrylate-based PSA which comprises 2 to 20 wt % of an N-alkyl-substituted acrylamide and 5 to 25 wt % of a (meth)acrylic ester having a linear alkyl radical having at least 12 carbon atoms, and 0.5 to 5 wt % of (meth)acrylic acid.
While such PSAs do have properties that are an improvement on the prior art, it has nevertheless emerged that the adhesive bond between the printing plates and the PSAs, which are per se reversibly bonding, becomes primed by impurities in the solvents with which the printing plates are cleaned after printing.
“Priming” or the “priming effect” in the present case is understood to mean that, as compared with printing plates cleaned with pure solvent, the bond strength of the PSA to the printing plates soiled by ink residues included in the solvents is significantly increased.
The impurities arise from ink residues from the printing inks in the solvents used for cleaning; even such small quantities of impurities that are not even visible are sufficient to bring about this effect. In this way, over time, significantly higher bond strengths are formed than is desirable for redetachment of the plates. In some cases the printing plates can be detached only with very high force application, as a result of which they may also be damaged, making it impossible for the plates to be used again. In order to avoid this, the printer is compelled to use fresh solvent and fresh cleaning cloths for each cleaning operation. Apart from the increased time and materials consumed in this case, it is hardly possible for this to be implemented in practice. Particularly because the soiling present is often not apparent to the eye, there is no acceptance among users to replacing solvents and cleaning cloths.
It is an object of the present invention, therefore, to provide a pressure-sensitive adhesive which in the bonding of printing plates, especially to printing cylinders and/or printing sleeves, as well as the aforementioned requirements, namely:                assurance of effective and reliable bonding to material common in flexographic printing such as, in particular, to PET (polyethylene terephthalate), even under the influence of solvents;        detachability even after prolonged time even from highly polar substrates such as, for example, the surfaces of steel printing cylinders or the surfaces of certain printing sleeves comprising polar plastic surfaces;        suitability for reliable adhesive bonding of printing plates; for an adhesive tape featuring the PSA, the stability of the adhesive tape assembly, especially the reliable anchoring of the PSA on foam carriers such as polyolefinic foams is to be ensured;also counteracts the unwanted priming effect of the printing-ink residues in the solvents with which the printing plates are cleaned, and which therefore exhibits so-called “ink resistance”, being insensitive to the influence of the printing-ink residues in the cleaning solvent. The bond strength of the PSA to surfaces soiled in this way may be equated with a physical anchoring whose cause may be attributed to at least one binder (also called film-former) present in printing inks. One binder, among others, commonly used in printing inks is cellulose nitrate (in common parlance “nitrocellulose”), which is able to develop interaction with the PSA on the bond area and hence is known to cause this primer effect.        